Finite-element modelling of the impact behaviour of aluminium nacre-like composite

Abstract

The demand for energy-absorbing lightweight structures for impact applications in automotive, aerospace and defence industry is rapidly growing, posing a challenge for innovative engineering design to maintain lightweight without reducing damage tolerance and impact and shock absorption. In this context, biological materials offer a source of inspiration for the design of new materials. Nacre, commonly known as the mother-of-pearl, is a biological material that exhibits outstanding mechanical properties due to its hierarchical structure, which includes a brick-like pattern, layer waviness and interface. Although nacre is made of 95% of aragonite, a brittle material, its toughness is about 3000 larger than that of aragonite. Research addressing the behaviour of nacre-like engineering composites is limited and this work intends to contribute to the understanding of such materials under impact loading. In this paper, the study of the impact behaviour of layered nacre-like plates made of 1-mm thick tablets of aluminium alloy 7075 glued with toughened epoxy resin is performed using Abaqus/Explicit. A 9-mm steel spherical projectile with initial impact velocities in the range of 400-900 m/s is used. The epoxy material is modelled using a user-defined cohesive element that accounts for the experimentally observable increase in both strength and toughness in compression. Target thicknesses of 5 and 7 mm are modelled. The ballistic performance of bulk plates made of bulk Al-7075 is compared with that of nacre-like composite plates of the same thickness. It is found that the nacre-like structures performed slightly better than the bulk plate for high impact velocities with a reduction of about 9% in the residual velocity; however, for lower impact velocities close to the ballistic limit, nacre-like plates performed worse than the bulk plate. The higher performance at higher impact velocities of the nacre-like composites is attributed to the hierarchical structure that enables both localized energy absorption by deformation of the metallic tablet and tablet interlocking due to the waviness and inter-layered delamination, which allows plastic deformation further away from the impact zone. It is concluded that nacre-like aluminium composite plates should be further investigated for their potential in designing protective structures because they could enable substantial improvements in weight-savings and in the ballistic performance of the structure. However, a quantitative assessment of their benefit warrants further numerical and experimental research.